1. Field of the Invention
The present invention relates to a zoom lens system for a compact camera, and in particular, relates to achieving a higher zoom ratio and achieving miniaturization of a zoom lens system.
2. Description of the Related Art
In a zoom lens system for a compact camera, in addition to requirements for miniaturization, a zoom lens system with a higher zoom ratio of 3.0 or more has increasingly been required in these years. Unlike a zoom lens system for a single lens reflex (SLR) camera, a zoom lens system for a compact camera does not require a longer back focal distance behind the photographing lens system. Accordingly, in a compact camera, a telephoto type zoom lens system including a positive front lens group and a negative rear lens group has been employed, while in a SLR camera, a retrofocus zoom lens system including a negative front lens group and a positive rear lens group has been employed for the purpose of securing a back focal distance.
In such a zoom lens system of a compact camera, if a zoom ratio of about 3.5 is required, a three-lens-group zoom lens system including a negative lens group, a positive lens group and a negative lens group has been conventionally employed. In this three-lens-group zoom lens system, zooming is performed by varying the distance between the first lens group and the second lens group. More concretely, in the prior art, in order to achieve a higher zoom ratio, the first lens group and the second lens group are generally moved so that the distance therebetween increases in a direction toward the long focal length extremity from the short focal length extremity. However, along with the increase in distance between the first lens group and the second lens group, there is a need to increase the diameters of the first and second lens groups in order to collect peripheral illumination. As a result, the overall length of the zoom lens system and the diameters of the lens groups have to be increased, so that miniaturization of the zoom lens system cannot be attained.
It is an object of the present invention to provide a miniaturized three-lens-group telephoto zoom lens system with a zoom ratio of about 3.5.
In order to achieve the above-mentioned object, there is provided a zoom lens system including a negative first lens group, a positive second lens group, and a negative third lens group, in this order from the object. Zooming is performed by moving the first, second, and third lens groups along the optical axis, and the zoom lens system satisfies the following conditions:
0.03 less than (dW12xe2x88x92dT12)/fW less than 0.1 xe2x80x83xe2x80x83(1) 
xe2x88x924 less than fT/f1G less than xe2x88x922 xe2x80x83xe2x80x83(2) 
wherein
dW12 designates the distance between the most image-side surface of the first lens group and the most object-side surface of the second lens group, at the short focal length extremity;
dT12 designates the distance between the most image-side surface of the first lens group and the most object-side surface of the second lens group, at the long focal length extremity;
fW designates the focal length of the entire zoom lens system at the short focal length extremity;
fT designates the focal length of the entire zoom lens system at the long focal length extremity; and
f1G designates the focal length of the negative first lens group.
The zoom lens system according to the present invention preferably satisfies the following condition:
1.0 less than f12T/f12W less than 1.2 xe2x80x83xe2x80x83(3) 
wherein
f12T designates the combined focal length of the first lens group and the second lens group at the long focal length extremity; and
f12W designates the combined focal length of the first lens group and the second lens group at the short focal length extremity.
Furthermore, the first lens group preferably includes a positive first sub-lens group and a negative second sub-lens group, in this order from the object, and the first sub-lens group satisfies the following condition:
xe2x88x921 less than f1G/f1a less than 0 xe2x80x83xe2x80x83(4) 
wherein
f1a designates the focal length of the first sub-lens group.
The positive second lens group can include a lens element in which at least one of the surfaces is made aspherical, and the aspherical surface preferably satisfies the following condition:
xe2x88x92800 less than xcex94IASP less than xe2x88x92300 xe2x80x83xe2x80x83(5) 
wherein
xcex94IASP designates the amount of change of the spherical aberration coefficient due to the aspherical surface under the condition that the focal length of the entire zoom lens system at the long focal length extremity is converted to 1.0.
The negative third lens group can include a lens element in which at least one of the surfaces is made aspherical, and the aspherical surface preferably satisfies the following condition:
0 less than xcex94VASP less than 0.4 xe2x80x83xe2x80x83(6) 
wherein
xcex94VASP designates the amount of change of the distortion coefficient due to the aspherical surface under the condition that the focal length of the entire zoom lens system at the short focal length extremity is converted to 1.0.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-184423 (filed on Jun. 20, 2000) which is expressly incorporated herein by reference in its entirety.